as MM said, negatives are an entirely different thing and a dedicated negative scanner is the way i went. i'm currently using one from here:
http://www.scanace.com/en/product/1800u.php . the price on the one i bought was only $129 u.s. it scans 35 mm slides and negatives ONLY. so, if you do want to go this route, make sure you get one that matches up to whatever size negatives you have. i've been pretty much happy with this unit. my only complaint was the software was a bit buggy.
also, if you only have one computer, current pc's running windows can only handle one type of usb scanner plugged into it. this is a known windows bug. so, my flatbed is on one computer and the negative scanner on another.
also, bear in mind that i bought this a while back and the same is true with my flatbed. the technology on flatbeds may well be such that you could do it ok on a flatbed now, so check around.
craig